Critically ill patients who are hospitalized and in either surgery or in intensive care wards often require continuous or periodic measurement of their arterial or venous blood gases and direct blood pressure. These measurements are used by the physician to assess the status of the patient's cardiopulmonary and metabolic systems. For purposes of accuracy and time dynamics, these measurements are made continuously or repeatedly with short periods of time between measurements. Current methods and measurement sensors require a direct interface between blood and the sensor.
In the case of the blood gases, which are PaCO.sub.2 (partial pressure of arterial carbon dioxide), PaO.sub.2 (partial pressure of arterial oxygen) and pH (hydrogen ion content), a needle and syringe is used to penetrate a peripheral artery (usually the radial or brachial) and a sample of blood (nominally 0.5 to 1 ml) is withdrawn. The sample is injected into the sample chamber of a commercial blood gas machine which exposes the blood to a series of electro-chemical sensors and displays the sensed values on the analyzer readout. Withdrawing a continuous series of blood from the body in order to obtain a continuous reading has never proved practical because of the trauma to blood cells experienced when blood is removed from the body and the tendency of the blood to clot and clog such systems. Also, in many patients the continuous withdrawal of blood or blood samples will bleed the patient beyond a safe level.
In the case of direct blood pressure, a fluid path is normally obtained by installing a small open plastic catheter into the appropriate artery via an introducer needle and guide wire technique (Seldinger method) or with a split needle (Luther method). The catheter is attached to a fluid line which is connected to the dome of a pressure transducer and vented to atmosphere during zeroing of the transducer. This fluid path is also connected to a continuous saline drip through the fluid system into the blood to keep the fluid path from clotting and thus degrading or destroying the pressure reading. Alternately, the fluid path is periodically flushed to dislodge any blood which may be clogging the catheter.